1. Field of the Invention
The present invention relates generally to a viscous fluid drive device. In particular, the present invention is concerned with a baffle wall formed in a cover of a viscous fluid clutch to support a rotatable control arm.
2. Statement of the Related Art
A thermostatically-controlled viscous fluid clutch for driving and rotating a vehicle cooling fan is well-known. A multi-bladed fan is removably secured to a body of the clutch. The fan and clutch assembly is installed between an accessory pulley of a vehicle engine (typically the water pump pulley) and a radiator. The clutch drives the fan at high speeds close to input speed when cooling is required and permits the fan to rotate at low speeds when cooling is not required. Thermostatic control of the fan through the clutch reduces the airflow noise caused by fan rotation and the load on an engine, resulting in horsepower gain and improved fuel economy.
A clutch plate, housed within the clutch, having lands and grooves is mated to the body having complementary lands and grooves. A pump plate divides the interior volume of the clutch into a pair of fluid chambers, a working chamber and a reservoir. Discharge ports in the pump plate permit selective flow of a viscous fluid from the reservoir to the working chamber into a shear zone formed between the lands and grooves of the body and clutch plate. Fluid shear in the lands and grooves transfers input torque from the clutch plate to drive the body and the attached fan.
When cooling is not required, the discharge ports in the pump plate are closed and the fluid in the shear zone is pumped into a pumping chamber. Return orifices in the pump plate permit passage of the fluid from the pumping chamber into the reservoir. The removal of a majority of the fluid from the shear zone substantially reduces the shear between the clutch plate and the body, thereby substantially reducing the rotation of the fan.
A control arm is placed adjacent the pump plate and is rotated to cover and uncover the discharge ports, thereby controlling fluid flow from the reservoir. Oftentimes, a bimetallic element is utilized to actuate the control arm. The bimetallic element may be connected to a control shaft which is in turn connected to the control arm. As the bimetallic element expands due to warm ambient air, the shaft rotates, thereby causing rotation of the control arm. As the ambient air cools, the bimetallic element contracts, causing the control shaft and the control arm to rotate in an opposite direction.
To accommodate manufacturing and assembly tolerances, and the expansion and contraction of a bimetallic element, the control shaft and the control arm are permitted to slide or "float" for small distances in conventional fan clutch assemblies. Such float or "end play" occurs along the longitudinal axis of the shaft. The end play of the control shaft permits greater machining and assembly tolerances, thereby reducing manufacturing costs. Furthermore, friction between the bimetallic element and a cover is reduced by the end play of the control shaft.
As the control shaft slides in a direction away from the pump plate, the attached control arm is pulled away from the pump plate and tends to wobble during rotation. When the control arm is in the closed position and pulled from the pump plate, leakage of fluid from the reservoir to the working chamber can occur at the discharge ports. Unwanted fluid can flow into the shear zone and create undesired high-speed rotation of the cooling fan when an engine is next started. Such high-speed operation creates unwanted airflow noise and can increase an engine warm-up period.
A condition known as "after-boil" can occur when an engine is shut off immediately after a stabilized, heavily-loaded drive (e.g., a long drive up a sustained grade pulling a trailer). Underhood temperatures in the engine compartment can result in a radiator core temperature which reaches the coolant boiling point. Such temperatures can cause the bimetallic element to react and rotate the control arm, thereby permitting fluid to leak from the reservoir to the shear zone. When the engine is next started, undesired high-speed fan rotation can occur.
When an engine is not running, fluid pressure may cause the migration of fluid from the reservoir into the pumping chamber through the pump plate return orifices and into the shear zone. When an engine is next started, undesired high-speed operation of the fan can occur.
The art continues to seek improvements. It is desirable that a viscous fluid clutch provide thermostatic operation of a fan when cooling is required. Furthermore, it is desirable that a clutch prevent the migration of fluid from a reservoir to the shear zone through pump plate discharge ports and return orifices when the engine is not in operation, thereby eliminating high-speed operation and unwanted airflow noise when an engine is started.